1. Field of the Invention
The present invention relates to a valve unit provided in a water mixing valve capable of selectively discharging either hot water at a high temperature or cold water at a low temperature, or discharging mixed water at an appropriate temperature after mixing hot water with cold water, and comprising a stationary disk and a rotary disk, each slidable against the other.
2. Description of the Related Art
A conventional type of valve unit for a water mixing valve is disclosed in, for example, Japanese Patent Laid-open Publication No. Hei 4-165170. The valve unit for the water mixing valve described therein comprises a stationary disk and a rotary disk. The stationary disk is provided with a hot water inlet, a cold water inlet, and a mixed water discharge outlet while the rotary disk is provided with a mixing chamber.
The rotary disk overlapping the stationary disk is slidably rotated so that mixed water at an appropriate temperature is prepared in the mixing chamber by regulating the flow rates of the hot water and the cold water, fed from the hot water inlet and the cold water inlet, respectively, and discharged through the mixed water discharge outlet.
In the case of the valve unit for the water mixing valve described above, a problem of slidable surfaces wearing away arises because the stationary disk and the rotary disk, in intimate contact with each other, are caused to slide each against the other. Accordingly, in the teaching of the invention disclosed in the Japanese Patent Publication referred to above, it is proposed that a hard carbon film be formed on either one or both of the slidable faces of the rotary disk and the stationary disk, making up the valve unit, in order to enhance resistance-to-wear of the valve unit.
The hard carbon film is a hydrogenated amorphous carbon film, called i-carbon film, or a diamond-like carbon film (DLC) because of its diamond-like structure and properties. Accordingly, it has high hardness, a low friction coefficient, and a good lubrication characteristic.
Enhancement in resistance-to-wear is therefore achieved by coating the slidable surfaces of the rotary disk and the stationary disk, making up the valve unit of the water mixing valve, with the hard carbon film.
However, the valve unit described above has a problem that when mounted in the water mixing valve and put to use, exfoliation of the hard carbon film from the surface of a substrate of the stationary disk or the rotary disk occurs because the valve unit is subjected to hot water at a high temperature and frequent sliding motion by a strong sliding force every time the water mixing valve is opened and closed. Thus, the water mixing valve described has a problem that it has been unable to gain high reliability for long term use.